The transmitting power amplifier of a wireless communication device is the unit that consumes the major amount of power in the wireless communication device. Thus, improvement in the power efficiency of the power amplifier is an important challenge for the development of the wireless communication device. According to recent wireless communication standards, improvement in spectrum efficiency using linear modulation is becoming a trend. Since an amplitude modulation mode has strict requirements for signal distortion, the power amplifier is required to operate in a high back-off (low input power) state where good linearity is obtained. However, operation in the high back-off state lowers the power efficiency of the power amplifier. For this reason, as an attempt to maintain the linearity of input and output signals while improving the power efficiency of the power amplifier, Envelope Elimination and Restoration (EER) has been under active research recently.
The EER is a technique designed to amplify an input signal (i.e., a modulated wave signal) including an Amplitude Modulation (AM) component and a Phase Modulation (PM) component with high efficiency. The EER technique restores an original waveform while linearly amplifying an input signal by extracting an AM component from the input signal, amplifying only a remaining PM component, and modulating the amplitude of the amplified PM component using the extracted AM component. FIG. 1 illustrates a configuration of a power amplifier of the background art to which the EER technique is applied.
As shown in FIG. 1, the power amplifier of the background art to which the EER technique is applied includes signal generating circuit 147, Radio Frequency (RF) amplifier 109, pulse modulator 104, driver amplifier 116, switching amplifier 105, low-pass filter 106 and band-pass filter 107.
Signal generating circuit 147 extracts an AM component from an input signal, and outputs the AM components as amplitude component signal 111 through terminal 145 to pulse modulator 104. Signal generator 147 also extracts a PM component from the input signal, and outputs the PM component as a phase component signal through terminal 146 to amplifier 109.
Pulse modulator 104 generates a square wave by pulse-modulating amplitude component signal 111, and outputs the square wave to driver amplifier 116.
Driver amplifier 116 drives switching amplifier 105 according to the square wave signal outputted from pulse modulator 104, thereby efficiently amplifying the square wave signal. After the amplification, the low-pass filter removes a spurious component from the square wave signal, which is in turn fed as a supply voltage through terminal 142 to RF amplifier 109.
RF amplifier 109 includes transistor 101, input power circuit 108 and output power circuit 140, and amplifies phase component signal 112 outputted from signal generating circuit 147. Here, an output signal of amplifier 109 is amplitude-modulated by an output signal of low-pass filter 106, that is, amplified amplitude component signal 114. In the meantime, a constant direct voltage is supplied from a power supply (not shown) generally through terminal 141 to input power circuit 108 connected to a gate of transistor 101.
band-pass filter 107 removes out-of-band components from a signal amplified using RF amplifier 109 (i.e., output signal 115), which is in turn supplied through terminal 144 to for example an antenna device (not shown).
FIG. 2 is a block diagram illustrating one configuration of the signal generating circuit shown in FIG. 1, and FIG. 3 is a block diagram illustrating another configuration of the signal generating circuit shown in FIG. 1. Signal generating circuit 147 shown in FIG. 2 is of an optimum configuration to be used when an RF signal is inputted to input terminal 143 of the power amplifier. Signal generating circuit 147 shown in FIG. 3 is of an optimum configuration to be used when a baseband signal is inputted to input terminal 143 of the power amplifier.
Signal generating circuit 147 shown in FIG. 2 includes amplitude detector 103 extracting an AM component from an input signal such as an RF signal and outputting an amplitude component signal and limiter 102 extracting an AM component from the input signal. Amplitude detector 103 extracts an AM component from an input signal (i.e., a modulated wave signal) inputted through terminal 143 and outputs an amplitude component signal through terminal 145. Limiter 102 removes an AM component from the input signal inputted through terminal 143 and outputs a phase component signal having a remaining PM component through terminal 146.
Signal generating circuit 147 shown in FIG. 3 includes baseband signal processing circuit 150 and Voltage Controlled Oscillator (VCO) 151. Baseband signal processing circuit 150 preferably includes a Digital Signal Processor (DSP) and a Digital/Analog (D/A) converter. Baseband signal processing circuit 150 outputs an amplitude component signal (i.e., an AM component) of a baseband signal (i.e., an input signal) through terminal 145, and outputs a signal for controlling an output frequency to VCO 151. Baseband signal processing circuit 150 calculates and extracts an AM component from the baseband signal inputted through terminal 143 by digitization using the DSP, up-converts the AM component, converts the AM component into an analog signal using the D/A converter, and then outputs the analog signal as amplitude component signal 111 through terminal 145. In addition, baseband signal processing circuit 150 calculates and extracts a PM component from the baseband signal inputted through terminal 143 by digitization using the DSP, generates a control signal for outputting a phase component signal from VCO 151, converts the PM component into an analog signal using the D/A converter, and outputs the analog signal to VCO 151. VCO 151 outputs an up-converted phase component signal on an RF frequency in response to the control signal from baseband signal processing circuit 150.
The power amplifier shown in FIG. 1 operates transistor 101 of RF amplifier 109 always in a saturated state by outputting phase component signal 112 with sufficient power from signal generating circuit 147. In addition, the power amplifier modulates the amplitude of phase component signal 112 amplified by transistor 101 using amplitude component signal 114 by supplying amplitude component signal 114 through terminal 142 and output power circuit 140 to the drain of transistor 101 of RF amplifier 109. Accordingly, the power amplifier can maintain the linearity of input and output signals while amplifying the input signal with high power efficiency. In addition, the power amplifier to which the EER technique is applied has been disclosed for example in Japanese Patent Publication No. 2005-184273.
Envelope Tracking (ET) is also known as another technique for improving power efficiency while maintaining the linearity of the power amplifier.
The ET is a technique designed to improve power efficiency while maintaining the linearity of input and output signals by amplifying an input signal including AM and PM components, extracting the AM component from the input signal, and modulating the amplitude of the amplified signal using the extracted AM component. FIG. 4 illustrates a configuration of a power amplifier of the background art to which the ET technique is applied.
As shown in FIG. 4, the power amplifier of the background art to which the ET technique is applied differs in the configuration and operation of signal generating circuit 148 from the power amplifier of FIG. 1 to which the EET technique is applied. Other constructions and operations are the same as in the power amplifier shown in FIG. 1 to which the EER technique is applied, and thus a description thereof will be omitted. In FIG. 4, the same or similar reference numerals as in the power amplifier shown in FIG. 1 are used to designate the same or similar components, except for signal generating circuit 148.
Signal generating circuit 148 extracts an AM component from an input signal, and outputs the AM component as amplitude component signal 111 through terminal 145 to pulse modulator 140. In addition, signal generating circuit 148 outputs modulated wave signal 149, proportional to the amplitude of the input signal containing AM and PM components, through terminal 146 to RF amplifier 109.
FIG. 5 is a block diagram illustrating one configuration of the signal generating circuit shown in FIG. 4, and FIG. 6 is a block diagram illustrating another configuration of the signal generating circuit shown in FIG. 4. Signal generating circuit 148 shown in FIG. 5 is of an optimum configuration to be used when an RF signal is inputted to input terminal 143 of the power amplifier, and signal generating circuit shown in FIG. 6 is of an optimum configuration to be used when a baseband signal is inputted to input terminal 143 of the power amplifier.
Signal generating circuit 148 shown in FIG. 5 includes amplitude detector 130 extracting an AM component from an RF signal (i.e., a modulated wave signal) and outputting the AM component as amplitude component signal 111. Amplitude detector 103 extracts an AM component from an input signal inputted through terminal 143, and outputs the AM component as amplitude component signal 111 through terminal 145. The input signal inputted through terminal is supplied to amplitude detector 130 and at the same time outputted as modulated wave signal 149 through terminal 146.
Signal generating circuit 148 shown in FIG. 6 includes baseband signal processing circuit 150 and quadrature modulator 152. Baseband signal processing circuit 150 calculates and extracts an AM component from the baseband signal inputted through terminal 143 by digitization using the DSP, converts the AM component into an analog signal using the D/A converter, and then outputs the analog signal as amplitude component signal 111 through terminal 145. In addition, baseband signal processing circuit 150 converts the input baseband signal into an analog signal using the D/A converter, and outputs the analog signal to quadrature modulator 152.
Quadrature modulator 152 up-converts the baseband signal outputted from baseband signal processing circuit 150 to an RF frequency, and outputs the up-converted baseband signal as modulated wave signal 149 through terminal 146.
The power amplifier shown in FIG. 4 has the function of limiter 102 of FIG. 2 in RF amplifier 109 by outputting modulated wave signal 149 with sufficient power from signal generating circuit 148 and thereby operates transistor 101 of RF amplifier 109 always in a saturated state. That is, the power amplifier to which the EER technique is applied and the power amplifier to which the ET technique is applied operate according to the same principle except that a PM component of an input signal is inputted into RF amplifier 109 in the EER technique but modulated wave signal 149 including AM and PM components is inputted into RF amplifier 109 in the ET technique. Therefore, the power amplifier to which the ET technique is applied can maintain linearity of input and output signals while amplifying the input signal with high power efficiency.
However, the power amplifiers of the background art as mentioned above have the following problem: As shown in FIG. 7, the power efficiency of switching amplifier 105 drops when the output power (i.e., average power) of switching amplifiers 105 shown in FIGS. 1 and 4 decreases. The power efficiency of switching amplifier 105 is dependent on the output power since the power loss of switching amplifier 105 with respect to the output power of switching amplifier 105 cannot be neglected in the low power mode.
The main reasons for the power loss of the switching amplifier include switching loss caused by overlapping current and voltage during switching from “on” to “off” or from “off” to “on”, or conduction loss caused by on-resistance. The dropping power efficiency of switching amplifier 105 also leads to a decline in the entire power efficiency of the power amplifying circuit.